Don’t know if it works for newer generation iPods, but this works with my 2nd gen. version.
In iTunes, set the ipod to work as an external hard drive.
Make hidden folders visible.
Browse your iPod folders. You will find a lot of folders with odd names that contain the music files. From memory, the folders have a letter and a number to name them. The files won’t have the full names, but they should be obvious from the file extensions.
*IMPORTANT* Copy these files to a new folder on your computer. Then, make a separate BACKUP copy of the whole iPod in a different folder, just to be safe.
Unplug your iPod.
Get iTunes to Add Files from a folder. Point the import tool to the root directory of the music files you copied from your iPod.
iTunes will rename the files, add artists, album names etc when you it adds the files.
This does not import playlists unfortunately.
Once you are satisfied that you have all the music on the new iTunes you can then sync the iPod.
If all else fails, you still have the music on your iPod.
Delete the files on you PC when you feel all is well.